1. Field of the Invention
The present invention relates to a fixing device for fixing a developer image formed on an image receiving medium by heating and pressurizing a developer image by fixing rollers after heating and fusing the developer image by a fixing belt and a heating volume regulating method.
2. Description of the Related Art
For a fixing device that is used in an electro-photographic type copying machine, printer and the like for heating, pressurizing and fixing a toner image that is formed on a paper, a heat roller type fixing device for simultaneously heating and pressurizing a toner image by inserting a paper between a pair of fixing rollers having a heater or a belt type fixing device for heating, pressurizing and fixing a toner image by inserting a paper between a pair of fixing rollers after sufficiently heating and fusing a toner image formed on a paper by an endless belt shaped fixing belt is used.
On the other hand, a full color electro-photographic type image forming apparatus has come into wide use in recent years. This type of full color image forming apparatus uses paper in a wide range of thickness and also uses OHP paper, etc. and therefore, a high fixing property is demanded in order for achieving good color reproducibility for paper having various kinds of characteristics.
In order to obtain good full color reproducibility on such various kinds of paper, a fixing speed of a fixing device is so far controlled according to characteristics of paper or a heating temperature of a fixing device is controlled according to the characteristic of paper as disclosed in the Japanese Patent No. 10-274903.
However, in the case of a fixing device that controls a fixing speed, if a fixing speed was retarded by about 50%, the number of sheets to be copied will decrease by half and the performance drops as a result of the decrease in the number of copies. Furthermore, when intends to control a fixing speed only while maintaining other image forming processes unchanged, a distance from a peeling device to a fixing device requires at least a length of, for instance, A 3 size, that is the maximum paper size and the size of a fixing device will become large.
On the other hand, in the case of a heat roller type fixing device to control a fixing temperature, when paper in the range of, for instance, 80 g/m.sup.2.about.209 g/m.sup.2 is usable, the paper thickness is classified into 4 kinds; ordinary paper of 80 g/m.sup.2.about.90 g/m.sup.2, thick paper 1 of 91 g/m.sup.2.about.105 g/m.sup.2, thick paper 2 of 106 g/m.sup.2.about.140 g/m.sup.2 and thick paper 3 of 141 g/m.sup.2 -209 g/m.sup.2 and a fixing temperature of the fixing roller is controlled at 4 stages of 155.degree. C., 165.degree. C., 170.degree. C. and 180.degree. C. according to the classification of paper thickness.
As a definite example, when intended to change the image forming mode to the image forming mode for ordinary paper (80 g/m.sup.2.about.90 g/m.sup.2) immediately after completing the copying in the image forming mode for thick paper 3 (141 g/m.sup.2.about.209 g/m.sup.2), the fixing temperature of the fixing roller must be controlled to lower by 25.degree. C. from 180.degree. C. for the thick paper 3 to 155.degree. C. for the ordinary paper. Normally, to lower a temperature of the fixing roller pair by 25.degree. C., a time of about 3.5 min. is required even if the temperature is positively lowered by rotating the fixing roller pair. Therefore, operator must wait for a long time until the fixing roller reaches a proper temperature, the operability is lowered and furthermore, in order to control the fixing roller temperature positively, the fixing roller must be rotated even during the non-fixing time, and the life of the fixing roller is shortened.
Further, to improve the full color reproducibility of a belt type fixing device, it is necessary to sufficiently heat and fuse a toner by the fixing belt before pressurizing and fixing the toner by the fixing roller. So, in order to give a sufficient volume of heat to a toner, it is required to surely control the temperature of the fixing belt or secure a sufficient heating/fusing time using a long fixing belt.
Therefore, when intends to surely control the temperature of the fixing belt, a waiting time until the fixing belt reaches a proper fixing temperature when changing over the image forming mode will become long and the operability is deteriorated.
On the other hand, when the fixing belt is arranged to overlap the paper conveying unit to the fixing unit to downsize the main body of the fixing device when a long fixing belt is used, it was necessary to drive the paper conveying unit in order to prevent the deformation of the paper conveying unit by the heating from the fixing belt even when no paper is conveyed. Furthermore, in the case of a fixing device in which the paper conveying unit and the photosensitive drum are driven synchronously, the photosensitive drum must be driven jointly with the paper conveying unit as long as the fixing belt is heated even during the warming-up. Thus, the life of the paper conveying unit and the photosensitive drum was shortened and cost was increased.
Accordingly, in the case of a belt type fixing device, when an image forming mode is changed over so as to get good full color reproducibility regardless of the characteristic of a paper, it is desirable to be able to obtain the good operability without requiring a long waiting time and furthermore, to extend the life of the fixing device. Further, in the case of a belt type fixing device, it is desirable that the life of the paper conveying unit and the photosensitive drum is extended without impeding the downsizing of the main body of the device when a long fixing belt is used so as to get good full color reproducibility.